Process for improving the adhesion of paint to wheel-abraded wood surfaces



United States Patent 3,450,554 PROCESS FOR IMPROVING THE ADHESION 0F PAINT T0 WHEEL-ABRADED WOOD SURFACES James W. Watson, Parma, Ohio, assignor, by mesne assignments, to SCM Corporation, New York, N.Y., a corporation of New York No Drawing. Filed Jan. 11, 1965, Ser. No. 424,838 Int. Cl. B44d 1/26 US. Cl. 117-57 3 Claims ABSTRACT OF THE DISCLOSURE scrubbed surface is then painted with conventional coatings.

This invention relates to an improved process for coating wood and more particularly to coating wheel-abraded plywood.

The abraded Wood surfaces concerned here display a decorative texture like that of weathered wood. Such texture resembles somewhat a very low-relief contour map in plywood. The texture is formed by the more rapid cutting and removal of the softer or spring wood sections by an abrading wheel simultaneously with the polishing and negligible to moderate cutting and removal of the harder wood portions of slower growth, i.e., summer wood. Such summer wood portions, after abrading, appear as dark ridges and raised planes on the wood surface, which ridges are denser, ostensibly more resinous, and often glossy. By abrading, I mean wheel-abrading with rapidly-rotating wire wheels that cut, typically, into spring wood up to about inch, the wood being thereafter sometimes referred to in the lumber trade as distressed or exhibiting a diftwood finish. When such resulting abraded surface is painted for decorative or other effect and subject to weathering, the paint will rapidly fail by crack staining, cracking, peeling, and/or checking, such failure usually becoming very obvious in a matter of 2-3 months.

I have now found a process for enhancing paint adhesion and thus retarding such peeling, etc., while also greatly reducing crack staining. It comprises scrubbing the abraded surface with hydroxylated volatile liquid solvent, and coating the resulting surface with paint. This process can be operated quickly and easily, readily lending itself to production line application.

My investigations have shown that an abraded wood surface has a vulnerable water-sensitive extractives content, which extractives interfere with good durability of paint adhesion on such surface and contribute to crack staining after painting. These water-sensitive extractives appear to comprise at least a portion of the extraneous components of the wood, e.g., terpenes, resin acids, sterols, fatty acids and aromatic compounds including the tannins, as well as comprising a portion of such major cell wall components as lignin and polysaccharides, tag, the phlobaphenes, coloring matters, lignans and the arabinogalactans.

In weathering of abraded and painted wood, moisture can reach these water-sensitive extractives, weaken and impair adhesion. Also, moisture moving through the coating surface and into the atmosphere, by way of the inevitable fissures which develop in paint will carry at least a portion of these water-sensitive extractives to the painted surface and deposit them on such surface upon evaporation of the moisture. This produces a discontinuously off-color surface, i.e., one that is crack-stained.

Noteworthy crack-stainers among woods, and thus those to which my invention is particularly suited are woods which are highly resistant to rot, such as cedars, redwoods and cypress, i.e., members of the families Taxodiaceae and Cupressaceae, such as western red cedar (Tlzuju plicata Donn.). However, I believe that my invention can be applied advantageously to abraded wood generally including distressed woods of the Doublas-fir genus. In the lumber industry woods of the Douglas-firgenus and also the wood of the western red cedar species are typically used in manufacturing plywood, and my invention is eminently suited to such plywood after it has been abraded.

I prefer to scrub the abraded wood surface immediately following the abrading operation particularly when the abraded wood is to be stored outdoors. Apparently when the wood is allowed to stand after abrasion, as for long periods, e.g., of six months or longer, in open lots exposed to sunlight, and precipitation, such conditions generally render the water-sensitive extractives less susceptible to removal by the scrubbing.

By scrubbing I mean subjecting the abraded surface to a solvent stream under pressure, preferably while employing some brushing of the surface. Alternatively, abraded wood can be immersed in a tub of solvent and then brushed. Suitable brush types include sponges as well as nylon or other synthetic bristle brushes, or those of vegetable fibers such as sisal, hemp or palmetto, and resilient wire such as brass which can be used in assisting removal of the water-sensitive extractives without causing substantial reabrasion of the wood. Where brushing is used it has been found advantageous to follow the brushing with rinsing of the wood surface with additional solvent to insure removal of the extractives prior to further processing of the wood.

The hydroxylated volatile liquid solvents which I can use are preferably liquids at room temperature (72 F.) and atmospheric pressure and boil not above about 250 F., also at atmospheric pressure. Advantageously, I use water, for efiiciency and economy. It can be acidified with hydrochloric, nitric, acetic or phosphoric acids or made basic as by ammonia solution, but that is not necessary. The water can be room temperature, boiling or cool: however, hot water often tends to produce a somewhat fuzzy wood surface of minute projecting wood fibers which is not the most desirable wood surface for good paint appearance. For this reason, and since it has produced the best results as well as being economical, I prefer to use cool water as my hydroxylated volatile liquid solvent.

Further solvents that I can and have used include methyl alcohol, butyl alcohol, ethyl alcohol, isopropyl alcohol, their mixtures, as well as aqueous solutions of same. For such reason, the water-sensitive extractives which are removed from the abraded wood in my process can be thought of as those which are also substantially sensitive to alcohol solvent.

After scrubbing, the abraded wood surface contains extraneous unbound solvent collections. Such collections can be generally regarded as the excess moisture contained on the scrubbed surface over and above the moisture such surface would exhibit when at equilibrium with the humidity of the surrounding atmosphere. Such solvent collections include the free solvent, i.e., droplets and puddles of solvent which will readily flow from the wood by simply turning the wood over or tipping it, as Well as including solvent films and similar collections which are not so simply removed. Before coating I advantageously remove the free solvent from the scrubbed surface to insure coating uniformity over the wood after application and drying of the paint. When the solvent used in scrubbing is not the same as, or miscible with, the dispersing medium of the paint, e.g., mineral spirits of oil-based paints or the water in latex paints, I prefer to remove sufficient quantities of the extraneous unbound solvent collections to provide a scrubbed surface which is dry to the touch, and will thereby provide a surface of best paint adhesion, as is well known to those skilled in the painting art.

Tipping of the wood or blowing air upon the scrubbed surface can be used to remove free solvent. When further extraneous unbound solvent collections are to be removed, e.g., to prepare an ostensibly dry wood surface, the wood may be simply air dried, but I prefer to blow warm air upon the wood which air, for efficiency, is heated above room temperature (72 F.). Infrared or other forcedrying technique can also be used.

I can coat by any suitable method for applying paint to a wood surface such as the conventional techniques which include airless spray, curtain coating, pressure spray, roller coating or reverse roller coating, dipping, rotary brush coating, or combinations of such methods such as spray and brush techniques. The selection of a particular method employed is well within the ability of those skilled in the painting art.

By the use of paints for coating, I mean to include the use of paints, enamels, varnishes and lacquers. Such paints can contain pigment in a binder or can be unpigmented, e.g., generally cellulose lacquers, rosin varnishes, and oleoresinous varnishes as for example tung oil varnish. The paints can be solvent reduced or they can be water reduced, e.g., latex or water-soluble resins, including modified or soluble alkyds, or the paints can have reactive solvents such as in the polyesters or polyurethanes. Additional suitable paints which I can or have used include oil paints, including phenolic resin paints, solvent-reduced alkyds, epoxys, acrylics, vinyls, including polyvinyl butyral and oil-wax-type coatings such as linseed oil-paraffin wax paints. The paints can be suitably applied as mill finishes, i.e., factory applied to the wood. Air drying as well as infrared or other force-cur ing method can be used for curing the paint. My invention is especially suitable for paints which are not waterreduced systems, because I have found generally that paint adhesion is then the most benefited and crack staining the most reduced.

The following example shows a way in which my invention has been practiced but should not be construed as limiting the invention.

EXAMPLE From western red cedar plywood panels which had been wheel-abraded by wire wheels which cut into the soft spring wood portions of the wood up to A inch, there were cut panels having abraded faces measuring 2 /2" x 6". Ten panels, selected at random, were placed under a constant flow of cool tap water (55 F.) while manually scrubbed with a palmetto brush. The brushing lasted for one minute, which was sufiicient to subject the entire abraded surface of the panels to the brushing. These ten panels were then rinsed for a few seconds under such tap water and allowed to stand exposed to the atmosphere until dry to the touch.

An additional 10 panels also selected at random were not so treated and were used for comparative purposes. All panels were then uniformly brush coated with the same latex paint and allowed to stand 24 hours, i.e., until all were dry to the touch. All panels were then placed in a weatherometer for accelerated testing.

The 20 specimens were mounted in the weatherometer on a circular rack 37%" in diameter, which rack rotated at 1 r.p.m. The speciments rotated around a carbon arc lamp which operated continuously at 50 volts and a current of 60 amps and provided uniform distribution of radiation on all specimens. Water was sprayed on the specimens under the following operating conditions: 18 minutes of light and water, 102 minutes of light.

In the following table the panels were rated according to the following scale: 10, perfect, e.g., the paint appearance resembled that of freshly applied paint; 9, excellent, with only slightly reduced gloss and/or some water spottin, no visible failures with surface cracks noted only by the aid of magnification; 8, slight overall color fading,

1 no gross surface failures, but some edge cracking; 7,

overall dull surface having further loss of luster, a satisfactory surface which has tiny surface checks with slight initial paint removal on ridges, i.e., summer wood portions; 6, exhibits poor luster with enlarged surface checks and increased paint removal at ridges, a barely satisfactory surface; 5, half of panel exhibits some type of failure.

Exposed hours in weather- Ten scrubbed Ten untreated ometer panels panels It will be seen from the above table that the treated panels not only resisted initial failure for a longer period than those panels which were untreated, but also failed more gradually, i.e., retained satisfactory appearance for the length of the exposure while the untreated panels were degraded to a marginal condition.

I claim:

1. In a process for painting a previously wheelabraded, unpainted wooden surface having water-sensitive extractives thereon produced by the abrading action which normally interfere with the adhesion to such surface of an after-applied paint, the improvement which comprises:

(a) removing said extractives by scrubbing said abraded wooden surface in the presence of a solvent selected from the group consisting of water and lower alkanols,

(b) removing from the scrubbed surface any free solvent present that is substantially immiscible with the after-applied paint, and

(c) subsequently painting the scrubbed surface for the first time.

2. The process of claim 1 wherein said wooden surface is plywood.

3. The process of claim 1 wherein the wooden surface is redfood, cedar, fir or cypress.

References Cited OTHER REFERENCES Vanderwalker, F. N.: Wood Finishing, Chicago, Drake & Co., 1944, pp. 18-20.

Hurst, A. E.: Painting and Decorating, London, Griffin 6 & Co., 1949, pp. 172-175.

WILLIAM D. MARTIN, Primary Examiner.

M. LUSIGNAM, Assistant Examiner.

US. Cl. X.R.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 ,450 ,554 June 17 1969 James W. Watson It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1 line 39 "diftwood" should read driftwood Column 3, line 70, "speciments" should read specimens Column 4 lines 7 and 8 "spottin" should read spotting same column 4, in the table, heading to the first column, line 1 thereof, "Exposed" should read Exposure same column 4, line 47, "redfood" should read redwood Signed and sealed this 31st day of March 1970.

(SEAL) Attest:

Edward M. Fletcher, Jr.

Attesting Officer Commissioner of Patents 

